Conventionally, hollow fiber membrane modules have been widely used in the field of so-called precision filtration applications including, for example, the preparation of sterile water, drinking water and high-purity water and the purification of air. In recent years, various attempts are being made to use hollow fiber membrane modules in highly polluted liquid treatment applications including, for example, secondary and tertiary treatments in sewage disposal plants, solid-liquid separation in night soil clarification tanks, separation of suspended solids (SS) from industrial waste water, direct filtration of river water in water purification plants, filtration of tap water for industrial use, and filtration of pool water.
However, most hollow fiber membrane modules used in these applications are of a cylindrical type which comprises a bundle of hollow fibers arranged in a circle or concentric circles and has conventionally been used in the field of precision filtration applications. Moreover, even where improvements are made in such hollow fiber membrane modules, they often comprise a mere modification of the packing density or packing form of hollow fibers.
When such a prior art hollow fiber membrane module is used for the treatment of highly polluted liquid (for example, SS&gt;50 ppm, TOC&gt;100 ppm), the hollow fibers cohere (or stick) together into a mass through the medium of the deposits (such as organic matter) accumulated on the membrane surfaces of the hollow fibers during use. As a result, the effective membrane surface area of the hollow fibers within the module is reduced to cause a rapid decrease in filtering flow rate. This phenomenon tends to occur especially in the hollow fibers located in the central part of cylindrical modules, and becomes more conspicuous with large-sized ones.
Although such a hollow fiber membrane module is periodically subjected to membrane surface cleaning or back washing, it is not easy to restore the function of the module whose hollow fibers have once cohered together into a mass, thus indicating a reduction in cleaning efficiency.